The present invention relates to door locks, and more particularly to an electrorheological fluid clutch for an electronic door lock.
Electronic door locks typically include a mechanical lock and an electronic control for authorizing the use of the mechanical lock. A portion of the mechanical lock secures the door to the door frame. The electronic control may include, for example, a reader that permits data to be read from a coded medium such as a magnetic card, proximity card, or memory key. When a card or key with valid data is presented to the electronic control, the control permits an outer handle or door knob to operate a shaft of the mechanical lock by actuating a prime mover to either release a latch that was preventing the handle or knob from turning, or engage a clutch that couples a shaft of the handle or knob to the shaft of the mechanical lock.
The mechanical lock and electronic control components (including the prime mover and latch/clutch) of electronic door locks are commonly powered by alkaline batteries which typically have a service life of between about two to three years. This limited battery service life necessitates changing the batteries several times over the service life of the door lock; a process that increases the operating costs of businesses which employ the electrical locks. Many prime movers, including most piezoelectric elements such as benders, exhibit capacitive characteristics such as a large inrush of power when initially electrically activated. This inrush of power operates as a short circuit load to the batteries, negatively impacting their battery life.
Electronic door lock latches incorporating a rheological fluid have been developed. One such latch utilizing rheological fluid is disclosed in U.S. Pat. No. 7,097,212 to Willats et al. Unfortunately, the Willats' latch suffers from drawbacks that affect the lock's performance and battery life. First, the rheological fluid in Willats is housed in a large cylinder which also has a piston disposed therein. For the Willats latch to operate, a sufficient current must be applied across the full cylinder to cause the viscosity of the rheological fluid to increase sufficiently to resist the movement of the piston. Because power consumption is directly related to the geometry (volume) of the contained rheological fluid, the use of the large cylindrical volume of fluid in Willats requires a relatively large inrush of power from the batteries. The Willats' latch also utilizes numerous moving parts including linkages and arms whose operation may be compromised by dust and wear. The moving parts and aforementioned cylinder make the latch rather large and bulky thereby necessitating that the latch be housed in an escutcheon rather than the door itself. The addition of the latch to the escutcheon may increase its size and thereby decrease the aesthetic appeal of the electronic door lock.